If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You will be required to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

HVAC design for a new house

09-22-2007, 06:38 PM

I've never posted here before - I'm usually in the woodworking forum.

Can anyone suggest a reference, web, books, or organization, that can advise on the design of a new HVAC system for a custom house (about 3500 sq ft)?? We are in the process of having plans drawn and are at the early stages. We live in AZ - where its HOT - so the HVAC design is very important not just for energy bills but for environmental reasons. I want to research this topic so I can evaluate and have input on the system the architect will design.

Any input or pointers to infomation on this would be much appreciated.

I havent had much luck with architects designing residential hvac systems. I would recommend finding a good hvac contractor that will do a full load calc off of your finished print. Let them design the system and install it.

Its important to take into consideration the recommended outdoor design conditions, as well as the indoor design conditions. The outdoor numbers are designed by the ACCA for your area. The indoor numbers are designed by you. (i.e. where are you going to set the stat) Tell me where in Arizona you are, and I will tell you the recommended outdoor design conditions for your area.

I'd be happy to answer any general questions relating to hvac, but for specific questions about system design related to your house I'd probably defer to a contractor in your area as we do things alot different here.

If you check out hvac-talk.com you might get lucky and find a contractor from your area who posts there.

Comment

Thanks! I live in east Mesa AZ, zip = 85207 which is part of the Phoenix metro area in the east valley.

I figured that architects probably would not design the best HVAC system. Two houses under construction we've looked at had to have reasonably major revisions made during construction. One was that the air handler was drawn to go in the atic space (flat roof), but there wasn't enough atic space, so it ended up going in a closet in the master cloths closet and reduced the closet size signifcantly!! The other issue was ducting - again there was insufficient space in the atic space (flat roof) for the ducts, so they had to run some of the duct work above the flat roof in full sun exposure. Then when I've looked at the plans of some houses and seen two air handlers in a one mechanical room on one side of the house, it doesn't make sence that one air handler should have duct work that travels to the other end of the house and for it to be so far from the air return.

Since we have not chosen a general contractor yet, as the plans will go to bid, we can't really refer the HVAC design to an HVAC contractor yet. I would be willing to pay someone to consult on the design, but its finidng someone who really knows their stuff. I've had some really bad luck with HVAC people here - not to digress too much - we had a unit replaced on a previous house. The bills were outragious even though we went with a higher SEER unit. It turns out that they left a large gap in the duct work so the unit was taking in air from the artic. I called them to talk about the huge bills and that the unit seemed to always run and they said "yeah thats normal sometimes". I had to fix all the leaks they left. I have more stories, but I digress...

Thanks for your help.

Comment

Contact some mechanical engineering firms. They will tell you they do commercial only most likely, but many have engineers who do residential design on the side. I know I did it for many years and now send all of my calls to a friend if its something I dont have time to do.

If I remember correctly and they are close, I have a friend who lived in Payson who has talked highly of Payson Air.

Regards,

Comment

I havent had much luck with architects designing residential hvac systems. I would recommend finding a good hvac contractor that will do a full load calc off of your finished print. Let them design the system and install it.

Its important to take into consideration the recommended outdoor design conditions, as well as the indoor design conditions. The outdoor numbers are designed by the ACCA for your area. The indoor numbers are designed by you. (i.e. where are you going to set the stat) Tell me where in Arizona you are, and I will tell you the recommended outdoor design conditions for your area.

If you check out hvac-talk.com you might get lucky and find a contractor from your area who posts there.

This is good advice, but let me add: Look in the area for a community college that teaches HVAC. Talk to the instructors. They "usually" know what is going on in the business and they must know how to do the load calculations; in fact this would probably be a good project for their sutdents.

Some of the other problems you are referencening are layout type problems. This can be solved by any competent duct and instilation guy , but many times the salesman or rep did not comunicate with the guy making up the duct work or the instalation team. It is not difficult work, but the left and right hands need to know what each is doing. This is usually the problem with engineers and architects, they each know their specialty but the architect did not notice that a 4'X3' furnace does not fit into a 2'X3' closet, etc.

Another idea might be to find a retired HVAC contractor or instaltion guy and pay him a small fee to look over everyone's shoulder, including yours.

Comment

This is usually the problem with engineers and architects, they each know their specialty but the architect did not notice that a 4'X3' furnace does not fit into a 2'X3' closet, etc.

This is funny cause my next piece of advice was going to be: Whatever the architect designs for space for mechanical rooms/chases, double it and that should be sufficient. More and more people are wanting this fiber optic ductwork youve heard so little about.

The outdoor design conditions for the Phoenix area are 107 in the summer and 38 in the winter. This means if you chose 70 as an indoor design temperature all the calculations would be based off of a 37 degree temperature difference in the summer, and 32 degrees in the winter.

Youve got all kinds of good advice so far. Let us know how the search goes.

Comment

Thanks for all the advice. I work at a Community College and will look up one of the HVAC instructors, if we have one, and get their input.

There are also several track home builders that build homes and given you a guarantee that your utility bills will not exceed a stated amount for 10 yrs. If I remember, they were saying that the bills on a 4500 sg ft home would not exceed $1500 a year, which is pretty good in Phx for that size house. If it did, they woudl pay the difference. They then say they have never had to it! Part of the build features they include were related to HVAC design which of I remember was to place the airhandler and ducts in the cooled space, not the artic.

I'm also surprised at the energy bills in my current house - 2700 sg ft 2 story track home - highest summer bills are $280. The HVAC seems to be a really well designed system. Except for the airhandlers and some duct work in the atic, the air return ducts and most of the ducts that feed the vents seem to be really short and many are in cooled space.

Thanks Dennis

PS If anyone can think of a book about efficient design, I'd be grateful for the reference.

Some of these books are for kids and some are too political for me, but I think these can get you started.

P.S. I would add a few things your builder will probably know about.

1) I really think a ridge vent for your roof is the best for attic ventilation rather than these solar powerd vents, etc. This will include proper ventilation throughout the attic as well, if this is a conventional home.

2) A radiant barrier in the attic, put in as you roof the house is great for A/C aid. The spray on kind is for existing homes, not new contrustion. This is good but not as helpful as the kind done as you do the roof construction.

3) Insulation is good, but at some point you have diminishing returns as opposed to the radiant barrier and vetilation. (See Tom Tynan on this, below.)

www.theremodelingshow.net Chis Miles a builder here has gotten into building "green" energy saving homes. He said at first he was very skeptical.

www.monlitiic.com The Monolictic Dome Institute. This is probably the most energy efficient construction I have seen, but it is not for everyone. The have done a school, I think, in Payson, AZ I know David South, you can trust what he tells you, a good man.

Comment

I just took a print that I figured about this time last year and I changed the location to Phoenix. My numbers here were 58,200btu heat loss, and 31545btu net sensible cooling gain. After changing the location to Phoenix, the numbers were 23,280btu heat loss, and 38,060btu net sensible cooling. This is with the front of the house facing NorthWest which puts the rear facing SE. This house has a considerable amount of glass in the rear.

The following are the net sensible cooling loads in btu's/hr with the house facing different directions.

North ~ 31,200
South ~ 28,088
East ~ 40,231
West ~ 40,391

As you can see there can be a huge difference in the amount of cooling your house needs depending on what direction the majority of the glass is facing. East and West loads are killer because you get direct sunlight in the mornings and evenings, which leads me to "Load Diversity". The east and west facing glass do not get their maximum loads at the same time, so therefore its not important add the max. values for each together. Also these net sensible numbers do not represent actual air conditioner size, only about 70%. About 30% will be added to these numbers for the Latent load. The minimum load for this house was with the rear facing North at 28,088. The maximum was 40,391 with the rear facing East. Thats a difference of 12,303 btu net sensible. Add your 30% for the Latent load and you are looking at just under 16,000btu. When selecting an air conditioner for this house, that would be a difference of 1 1/2 ton just by turning the house 90 degrees on the lot.

Hopefully this can help your design a little.

Comment

Thats amazing. Thanks for these figures. I'm not clear exactly what they mean, but I get the point. Although our orientation on the lot is not very flexible, we do plan on taking exposure into account when placing and sizing windows and adding covered patios. Those numbers really are amazing. Thanks for taking the time to do this.